Fix for duplicate arguments bug 117

[iWiggins]

Fixes #117 by searching the tree for duplicate arguments on parse.

Normal context throws a ParseError if a duplicate is found.
Noexcept context sets a UsageError if a duplicate is found.

[Taywee]

The concept is fine by me, but the tests don't look right. Each executable looks like it defines 6 tests, but only calls two of them. The testDuplicateShortInTwoGroups examples actually duplicate long arguments (even though they aren't called).

The extra tests look useful. Those should be fixed and enabled.

[iWiggins]

@Taywee Enabled the missing tests.

[iWiggins]

Reworked the detection to occur when a flag is added, instead of on parsing.

[Taywee]

Another implementation of this fix, #189, was recently opened as well. That looks like it might be a simpler implementation, but I don't see any evidence that it works across subgroups, and the testing here is more complete.

Edit: Actually, it does look like that one will probably work across subgroups, it's just not tested for yet.

[Taywee]

I poked at #189 a bit, and tried an approach merging this one and that one, and it looks like it's simple because to extend it to handle error-on-add, it would have to look pretty much exactly like this one.

[Taywee]

After these changes, I'm happy with the shape of it. Thanks for interfacing with me, and thanks for the PR.